Translating device and method of making it



948. OHL 2,437269 TRANSLATING DEVICE AND'METHOD OF MAKING IT 3 Sheets-Sheet 2 Filed April 10, 1944 /NVENmR R 5 OHL ATTORNE 4 armn 9, 1948. R. s. OHL 2,437269 TRANSLATING DEVIE AND METHOD OF MAKING IT Filed April 10, 1944 3 Sheets-Sheet 3 Fm; a

!NI/EN TOR BY R S 0 .WWW

ATTORNEY tremely cliicult in practice] metallurglcal methods to eliminate all traces of impurities and obtain ingots of chemically pure material. Applicant's method, thereore, makes it possible to prepare the silicon elements or wafers from available high-purity silicon material and to form on the surface thereof an extremely thin integrai high-impeclance layer having the desired rectification properties. This thin surface layer has the relatively high electrical impedance essential to the desired recticatior performance, yet the thicimess ot' this surface layer is such a minute percentage of the thickness of the waer or element that its efiect on the total impedance is small,

another feature of the invention is the method of making a translation element in which a highimpedance layer of desirecl thickness is formed on the rectification surface of a body, from which it is derived, composed of silicon containing a small percentage of impurities.

Another feature is a translating device includa body of silicon containing a small percehtage of impurities and having a thin crystallhe layer oi predeterminccl thiclness at the surilace thereof and a fine contact element making a contact with the surface of this crystal line layer.

These and other features of the invention will be described more fully in the following detailed specification.

?in the drawings accompanying the specicamon:

is an ehlarged View oi a block of fusecl f a high degree of purity: slah cut from the block of Fig. l;

' illustra es a mechanisn for polishing the V of the silico slabs;

l g. is a furnace for subjecting the silicon slabs 'for heattreatment;

Fig. is a cross-sections] View, greatly enlarged, ilir the; the strata formed during the heat ut oi' the slab;

Fig. is an enarged View of the slab illustrating the acid treatment for removing the surface cmst;

'Z is an eniarged assembly view of the translator;

Figs. 8 and il are comparative Operating ch acterstics; and

ro i'lustrates one useful application for the ti hsiating clevice.

As ailudecl to above there are definite limiteto the curreht-carrying Capacity of the point-contact slicon rectiers now available ior use in the microwave field. The reason for this Iimitation relates to the action that takes place the point of contact between the silicon Crystal and the fine tungsten Wire engaging the surface thereot'. If We assume that the silicon Crystal is of the positive type, it follows that the flow of current through the rectifier results from the passage of a stream of electrons from the tungster wire to the slicon element. So long as the transfer between the tungsten Wire and the silicon element is comprisecl of electrons only no physical impairment results. However, any attempt to in ore se the current ow above the limiting value resuts in the transfer of tungsten atoms as well as eieotrons to the silicon Crystal. This con taminates the surface of the silcon and soon impairs the eciency of the rectlfier.

In calculating the current limitation above mentioned it is assumecl that the mean distance hetween the tungsten point and the silicon surface is very small, that is. much less than the mean free path of an electron in air. Electron current, therefore, will flow across this short gap as though it were in a high vacuum.

The velocity of an electron in vacuum is given y tutv?? E. S. U

Where V is the applied voltage. eis the electronic charge, m is the mass of the electron.

The current owing across the gap is 275 z= neuntm/ (2) Where n is the maximum number of electrons available at the tungsten surface and is equal to the number of tungsten atoms at the surface layer multiplied by the number of valence electrons. which is a maximum of 6 for tungsten.

Therefore, n equals e multiplied by the number of atoms per square centimeter multiplied by the area of the contact point, or

n=6NA The approximate number of atoms per square centlmeter can be determined from the radius of a neutral tungsten atom which is given as 1.37 angstroms. Thus 10 LM 1.3? eine ttr] (4) The measured diameter of the tungsten contactng area of rectiers of the type above mentioned was found to average about &lxwcentimeters. Thus for a representative specific calculation Substituting the value of ?a in (2) and inserting the constants. the maximum current becomes Where V is the voltage in volts, across the boundary of the contact area.

The value of V can not be measured directly because of the potential drop in the silicon itself, although it can be determined indirectly from the current-voltage curve. The point, no doubt, will have unequally divided currents due to surface irregularities. However, neglectirg the possibility of non-uniform current distribution. the greatest expected current which can flow at 2 volts will be about 78 millarnperes. Actually the unts show signs of impairment at lower current values, which is to be expected especially if the current density at the boundary is not uniform.

The significance of the foregoing calculation is that the power-carrying Capacity oi' the pointcontact rectifier cannot be increased much, if at all, by causing the points to Carry more current. Furthermore, the rectiflers now available also have denite voltage limitations. This is illustrated in Fig. 8, which shows the current-voltage characteristic of silicon type rectifiers now in use. From this figure it will be seen that the unit commences to pass current in the reverse direction in response to relatively small values of applied voitage, and these negative values, of course, i

seen that current does not begin to flow in the reverse direction until a large value of voltage is applied. The advantages of this characteristic will be explainecl hereinafter.

'n accordance with applicant's process the inherent impurities which occur in very small percentages throughout the body of a silicon slab cast from available high-purity silicon, are substantlaily excluded from the extremely thin highimpedance layer formed on the surface of the slab bythe heat treatment. Because of the inherent nature of the element silioon and the compounds from which it is derived, it is practically impossible, by the methods now in use, to obtain one hundred per cent purity in elements of crystalline silicon of the sizes suitable for use in rectifiers. There are a susbtantial number of substances most of them elements which consistcntly appear as impurities ina body of highpurity silicon. Elements cut from ingots of the high-purity silicon commercially available are known to have electrical properties which make them suitable 'for use in rectifying devices. Although the body of one of these elements is of small dimensions it must be traversed by the current owing in the rectier, and the value of this current 'is therei'cre dependent upon the electricai resistance of the silicon body To make the element oi chemically pure silicon, even if practicai methods were available for this purpose, would increase its resistance to a prohibitive value. accordingly, the equivalent of this desirabie end is achieved through the present invention by iorming on the surface of the element a layer having the requisite high impedance cl'aracteristic and. yet too small in its dimension to seriously aiect the impedance offered to the signal current under-going rectification. The process by which this suface layer is formed includes a number of steps which will now be explained;

The first step in the process is to prepare an ingot of crystallized silicon of the positive type. The ingot is prepared by iusing powdered silicon of a high degree of chemical purity in an electric 'furnace under carefully regulated conditions. One suitable method of fusing these high-purity silicon ingots is disclosed in the application of J. Scai'f, Serial No. :386,835, filed April 4, 1941, Patent Number 2402582, datecl June 25, 1946.

after the iusion step, the ingot is sha'ped into a block i of desirecl dimensions. From the block i a'slab *2 cut by'means of a diamond saw. Whiie the dimcnsions of the slab 2 may be varied to suit the requirements, it may be explained that these slabs 'may have a thickness oi' l or 2 millimeters and a sidedimension of l or 2 centimete'z's.

Next the two large faces of the slab are given a preliminary smcothing ona cast-inoti lap with some suitable abrasive. One of the fiat surface-s of the slab 2 is now fixed to the disc 3 by means ofshellac or other adhesive material. The disc 3 is then lowcred into a polishing bath i. until the other flat surface of the slab 2 rests upon the surface of the polishing lap 5. The lap 5, which is preferably of tin, is provided With a series of V-shaped concentric grooves and is rotated by the shaft 6. The sha'ft i, to which the disc 3 is attached, rotates in the same direction as the shaft E but at a. different speed and is also arranged to participate in an oscillatory movement as indicated by the arrow. The bath 4 consists of any suitable liquids and abrasive materials. By rotating the disc 3 and lap 5 in the same direction at different speeds and by introducing the oscillatory movement of the disc 3 the surface of the slab 2 is given an extremely smooth finish o' optical polish. 'In fact, when polished in this manner, it is found that the whole face of the slab does not vary more than :Vi wavelength of green light and is flmost perfectly free of any signs of scratches.

The next step in the' process is to subject the poished slab to a heat treatment To this end it is detached from the disc 3, cleaned, and placed in'the electric furnace 8. The temperature of the heat chamber is carefully regulated I by thermostat 9 and is held at 1050 C., for the desired length of time. A suitable atmosphere containing oxygen is maintained in the heat chamber throughout the period by means of inlet and outlet pipes ao and ii and by suitable external controlling apparatus.

'The effect of heating the slab 2 under these conditions is to cause the formation of a vitreous layer or crust of silicon dioxide, mingled with crystalline aggregates of silicon, over the polished surface. This oxidized layer i, shown highly magnified in Fig. 5, is derived by the chemical reaction between the silicon atoms from the body of the slab'and the oxygen atoms in the chambe' atmosphere. The silicon molecules move up through the body of the slab and concentrate at the surface Where some of them immediately combine with the oxygen atoms and form silica, which deposits on the surface. As the silica layer i? develops in thickness migrating silicon molecules continue to pass up through the silica layer until they reach the surface and join With the oxygen atoms in the chamber atmosphere. a portion of the uncombined molecules formim; crystalline aggregates of the silicon which mingle with the silica. The concentration of these aggregates increases in the silica layer as the surface is approached. Also the impurities occurring near the surface of the slab, and particularly the oxides of metal impurities, tend to diffuse into the silica, layer formed on the surface, thus decreasing the amount of impurities in the layer immediately beneath the silic-a covering. The eifect, therefore. of this process is to form at the polished surface of the slab immediately under the over-lying vitrcous crust of silica. a thin integral layer of silicon from which substantially all of the inherent inpurities of the material are excluded. The important characteristics of this surface layer are, as above noted, its high elec trical impedance and its excellent rectification performance. I

As above noted. the optimum temperature for producing this cilect is around 1050 C. At thi:: temperature the iormation of the high-impedance layer takes place at the surface of the slab without impairing to any great extent the optical asetet@ finish which was gven the polishihg process. At higher temperatures the silicon molecules appear so rapidly that buhhles form and physical irregularities cccur, imparing the finish of the surface and rendering the slab unsuitable for best rectication performance.

The thickness of the crystalline layer at the ootlcal surface of the slah and the thickness of the overiylng vitreous crust are functions of the time of the heat treatment. As the heat treatment progresses in time, the crystalline layer becomes thicher mel thicker and likewise the excess of silicon molecules at the surface comhincs with oxygen to progresslvely increase the thickness of the vltreous 'overlying crust The time factor of the heat treatment, therefore, :hay be utilized to control the electrical cl'aracteristics oi* the slicon slab. As an illustration of the time elements involved, appllcant has :found that a heat treatment of about four hours gives rectiying elements having excellent cha'acteristics for certain uses. The layers thue iormed are illustrated roughly in Fig. 5. The thi layer iii represents the slicon crystal concentratioi at opticai surfwe of the slah 2; and overlying layer i?? of silica is considerably The next step in the process isto remove the vitreous layer ccvering the surface of the hoat treated slab to expose the surface layer. To this end the siab is immersed in a bath of hych'ofiuoric ac'd solution in a was: container l'l. The which the vitreous layer i? is removed depeicls upon the co'scentration of the so mon. s an iiustratioh t has been that coecehtrations from 5 to 20 per cent e' lsjfacto results. The length of the acid treatment period not particularly critical since :sot edversely afi'ect the crystalline once the vitreous layer 92 has been seid treatment completed the slab removed from the bath and cleahed and J cutting and mounting in the assembies. As above mentioned, the r, tiyig materia thue produced comprises a body E? of suhstantlal thickriess consistlhg of silicoa a high degree of urity but containing a small percentage of impurities, including metallic elements such as alumizum and iron, a a thiri uroi-orari high-impedance surface of crystals with a finish that is extremely si'icoth.

The i' surface of the slab is how electro- ;olated with such as nickel, and the slab is cut small. individual elements or waters. C ne of these waters, for example the wafer is then soldered or otherwise aflixed to the stud i@ of the metallic base i@ (Fig. i). The stud is now screwed into the ceramic cylineier t. In a similar manner, the stucl which is integral with the cap 22, is firmy screwed into the opposite end of the cylnder' The cap contains a central bore for receivihg the cylindrical contact holder 23. The holder is edjusted until the tip end of the tmgsten contact wire 26, the cpposite end of which is eoldered into the 'holder 23, makes contact the prepared surface of the wafer 323. a desired degree of force has been applied to the contact engagement of the wire i with the silcon wafer, the set screws 25 are tlghtenecl to sclze the holcler As above noted, the high power-carrying capacity of this rectler makes it especially suitable for a number of applications. One such use, illustrated in Fig. 10, is the generation of harmonic waves. It is well understood that a generating circuit which is designed to suppress the flow of current throughout a large part of the cycle of the applied fundamental wave and to permit its flow for a brief interval during the more efiectlve portion of the wave has an output rich in waves of harmonic frequencies. This is particularly true if the generating clrcuit is capable of passing current during that portion of the 'undamental cycle when the wave is in the region of its maximum amplitude. With this well-understood prlnciple in mind, it will now ice seen from the characterstlc curve shown in Fig. 9 that applicant's rectifier is admirably suited for the generation of harmonic frequencies m Fle. 10 a. simple harmonc generating circuit is illustrated including a source of fundamental frequency Zt, a rectifying unit 27, a load resistance 28, a condenser 29 and abiasng circuit 38. The biasing battery 38 is poled in such a manner that it applies a negative biasing potential to the rectifier 2?. The effect of this biasing voltage is to fix the zero line of the applied fundamental Wave at the proper point along the fiat portion 32, of the characteristic curve. In view of the relatively large values of negative voltage required to cause current to flow in the reverse direction, it is possible to adjust the zero line of the fundamental wave in such a position that the rectier does not pass current on the positive half-wave y until near the maximum amplitude thereof. And

this, as was explained above, is the condition most favorable to the generation of harmonia frequencies..

What is claimed is:

l. The method of 'making a translating device for electric waves of high requency which comprises forming a crystalline body of silicon having definite electrical characteristics and heat-treating said body to form on the surface therect a layer having electrcal characteristics which difier from those of the remaining part of said body.

2. The method of making a translating device for electric waves of high frequency which comprises forming a crystallne body of silicon having clenite electrcal characteristics and subjecting said body to heat treatment to form on the surface thereof a thin integral layer of material having electrcal characteristics which differ from those of the remaining part of said body.

3. The method of making a translating device for electric waves which comprises formng a body of crystailine silicon having a definite electrical impeclance, and heat-treating said body to form on the surface thereof a layer having a relatively high electrical impedance.

4. The method of making a translating device for electric waves which comprises forming a crystalline body of silicon, heat-treating said body to form on the surface thereof a thin layer of high-resistarce material, and fixing the duration and temperature of said beat treatment to control the thickness of said surface layer.

5. The method of making a translating device for electric waves which comprises forming a crystalline body of silicon having definite electr'cal characteristlcs and heat-treating said body to form on the surface thereof a thin layer of material derived from the body and having a, high electrlcal impedance.

6. The method of making 'a translatin devicc for electric waves which comprises forming a crys- &somos tallin@ hoy ot sillem hag a all percentage of nhorcnt amour-mes thereln, anal formim on the surface or said body a thin layer of highimpcanco material from which said impurlties are snhstantlally excluded. 7. The metho of making a translatng device for electrc waves which comprises ormng a. body of slllcon of high purity, subjectlng said body to hoat at a preetemtned temperature 'to cause the movement of molecules to the surface of said by where a portion of them oxldlze to form a coatm; of slllca over said surface. the unoxldized molcculas romina a crystalhne layer at said surface and removing said coating of sillca to ex nose said crystalhne surface layer.

8. The method of mama a translating devlce for slectrlc Waves which oomprlses orming a ma of 51116033 containing a small percentag of of simson having a high egree of put-its', pollshng the surface of sad body to an optlca finish, subjcctlng salc? body to hoat in an atmosphere of oxygen to cana@ the formaton over the surface tharco of a vtrcous layer of sllica and the for- :nation at said pollshed surface of a thin crystalline layer, controllng the rate of oxidation by regulating the temperature, controling the thickness of said layer of crystals by the duraton of the heat treatment, and removing said vtreous layer by an acid treatment to expose the pollshed surface of crystals.

10. 'I'heymethod ot making a translatng devce for electrlc waves of high l'requency which comprlses formlng abodyof silicon of high purity, polishirg a surface of said body to a high degree of smoothness, subjecting said body to heat at a temperature of 1050 C. to form a coating of sillca over the polshed surface and a thin crystallne layer at the polished surface, and removing said coating of silica by treatment with hydrofiuorc acid to expose the poished surface of crystals.

11. A translator for ultra-high frequency elect'lc waves including a body of sillcon containing impurlties and having a thin integral layer of high electrica impedance at the surface thereof, and a 'fine conducting element making a. point con tact with said surface layer. y

12. A translator for ultra-high frequency electric waves including a body of sllicon of a high degree of purty said body having a highly polished surface and a thin uniform crystalline layer at said polished surface, and a fine conducting elew ment making a point contact with said Crystal layer. v

v RUSSELL S. OHL. 

